Adapt build status icon for ci.jenkins.io permissions change (#764)

Large language models and other readers were causing performance problems.
The ci.jenkins.io controller has been reconfigured to limit access to
authenticated users in order to prevent performance problems.  That change
requires that we adjust the URL of the embeddable build status update URL.

Testing done

* Validated the change in a sample repository
* Will check the pull request after it is submitted

Author: MarkEWaite

README.md

@@ -1,5 +1,4 @@
-[![Gitpod ready-to-code](https://img.shields.io/badge/Gitpod-ready--to--code-blue?logo=gitpod)](https://gitpod.io/#https://github.com/jenkinsci/gitlab-branch-source-plugin)
-[![Build Status](https://ci.jenkins.io/job/Plugins/job/gitlab-branch-source-plugin/job/master/badge/icon)](https://ci.jenkins.io/job/Plugins/job/gitlab-branch-source-plugin/job/master/)
+[![Build Status](https://ci.jenkins.io/buildStatus/icon?job=Plugins%2Fgitlab-branch-source-plugin%2Fmaster)](https://ci.jenkins.io/job/Plugins/job/gitlab-branch-source-plugin/job/master/)
 [![GitHub release](https://img.shields.io/github/release/jenkinsci/gitlab-branch-source-plugin.svg?label=release)](https://github.com/jenkinsci/gitlab-branch-source-plugin/releases/latest)
 [![Gitter](https://badges.gitter.im/jenkinsci/gitlab-branch-source-plugin.svg)](https://gitter.im/jenkinsci/gitlab-branch-source-plugin)
 [![Jenkins Plugin Installs](https://img.shields.io/jenkins/plugin/i/gitlab-branch-source.svg?color=blue)](https://plugins.jenkins.io/gitlab-branch-source)